hydra

(often initial capital letter)Classical Mythology. a water or marsh serpent with nine heads, each of which, if cut off, grew back as two; Hercules killed this serpent by cauterizing the necks as he cut off the heads.

any freshwater polyp of the genus Hydra and related genera, having a cylindrical body with a ring of tentacles surrounding the mouth, and usually living attached to rocks, plants, etc., but also capable of detaching and floating in the water.

a persistent or many-sided problem that presents new obstacles as soon as one aspect is solved.

(initial capital letter)Astronomy. the Sea Serpent, a large southern constellation extending through 90° of the sky, being the longest of all constellations.

1835, genus name of a freshwater polyp, from Greek Hydra, many-headed Lernaean water serpent slain by Hercules (this sense is attested in English from late 14c.), from hydor (genitive hydatos) "water" (see water (n.1)); related to Sanskrit udrah "aquatic animal" and Old English ottur "otter." Used figuratively for "any multiplicity of evils" [Johnson]. The fabulous beast's heads were said to grown back double when cut off, and the sea creature is said to be so called for its regenerative capabilities.